In PET imaging, positrons are emitted from a radiopharmaceutically doped organ or tissue mass of interest. The positrons combine with electrons and are annihilated and, in general, two gamma photons which travel in diametrically opposite directions are generated upon that annihilation. Opposing crystal detectors, which each scintillate upon being struck by a gamma photon, are used to detect the emitted gamma photons. By identifying the location of each of two essentially simultaneous gamma interactions as evidenced by two essentially simultaneous scintillation events, a line in space along which the two gamma photons have traveled (a “line of response,” or “LOR”) can be determined. The LORs associated with many million gamma interactions with the detectors are calculated and “composited” to generate an image of the organ or tissue mass of interest, as is known in the art.
Some of the earlier PET systems used monolithic scintillation detectors—i.e., detectors that were each made from a single, unitary crystal element—and photosensor elements (e.g., photomultiplier tubes (PMT's) or avalanche photodiodes (APD's)) coupled to them to detect the incoming gamma rays and generated scintillation photons, respectively. In such systems, the resolution of the system—i.e., the ability to localize the interaction event—was limited by the size and hence “packing” or “clustering” ability of the photosensor elements. Therefore, to improve the resolution, subsequent systems were constructed with pixelated scintillation detectors, i.e., detectors that were comprised of a multitude of much smaller, cubic scintillation elements. Because the scintillation elements were so much smaller than the photosensors, the gamma interaction and scintillation photon generation could be localized with much better accuracy. Fabricating pixelated scintillation detectors is, however, relatively labor intensive, and there is a limit to how small the scintillation elements could be made.
Furthermore, the known pixelated gamma detectors typically could be used only to determine the location of gamma interaction with the detector in two dimensions, which gave rise to parallax errors. More particularly, a conventional two-dimensional measurement of the spatial location of a detected gamma ray absorption event in the scintillating crystal is limited to a two-dimensional point in the X, Y plane of the crystal. However, because the number of scintillation photons reaching each photodetector element is dependent on the solid angle subtended by the area of that detector element to the point of the gamma ray absorption within the crystal, the amount of scintillation photons received by each detector is also a function of the depth of interaction (DOI) of the incident gamma ray within the crystal, i.e., along the Z axis of the crystal.
The DOI is an important parameter when applied to imaging detector geometries where the directions from which incident gamma rays impinge upon the crystal are not all substantially normal to the crystal surface. If incident gamma rays intersect the crystal from directions not normal to the crystal, the unknown depth of interaction of those gamma rays within the crystal will result in an additional uncertainty in the measured position of the interaction because of the parallax effect, if only a two dimensional (i.e., X, Y) spatial location is calculated for such an absorption event. A detailed explanation of the importance of and the problems associated with the DOI is provided in “Maximum Likelihood Positioning in the Scintillation Camera Using Depth of Interaction,” D. Gagnon et al., IEEE Transactions on Medical Imaging, Vol. 12, No. 1, March 1993, pp. 101-107, incorporated herein by reference.
Thus, it is desirable to provide a gamma detector that has the relative simplicity of construction of a monolithic crystal detector but that affords the improved resolution associated with a pixelated gamma detector. Moreover, such a detector suitably is configured such that parallax errors could be reduced by using depth of interaction (DOI) information to increase the spatial resolution of the system, i.e., to provide the location of gamma interaction in three dimensions in space.